We describe generation of 1.1 W of 257 nm emission by frequency quadrupling the 1030 nm emission from a compact passively Q-switched Yb:YAG laser. The laser utilized a volume Bragg grating to achieve a 0.1 nm linewidth required for UV-Raman spectroscopic applications, generated 100 kW peak power, 250 μJ pulses and 3.6 W of average power at 1030 nm. Fourth harmonic generation (FHG) was carried out using a 10 mm lithium triborate (LBO) crystal to generate 515 nm second harmonic with 70% conversion efficiency, followed by a 7 mm beta-barium borate (BBO) crystal to generate 257 nm fourth harmonic with 45% efficiency, resulting in an overall nonlinear conversion efficiency of 31%. Far-field and near-field of the FHG emission were characterized.
© 2016 Optical Society of America
Laser-based sources of deep ultra-violet (UV) emission are required for several applications, including Raman spectroscopy based identification of materials and remote detection of explosives [1, 2], material processing, and photolithography. The most common techniques for generation of UV below 300 nm rely on frequency quadrupling of laser sources operating in the near-infrared band. Various implementations of this approach included frequency quadrupling of emission from laser diodes , Nd:YAG, Nd:YVO4 and Yb:YAG diode-pumped lasers [4–11], and Yb-doped fiber lasers [12–16]. In this work we targeted the development of a compact deep UV source that could be used in portable Raman spectroscopy systems for stand-off detection of explosive materials and their chemical precursors. The deep UV band has important advantages for Raman detection of explosives: increase in the Raman cross section that scales as the fourth power of the excitation frequency , and operation in the solar blind region to eliminate detector noise caused by solar background. Depending on the specific detection method and standoff range, UV powers in the range of 50–1000 mW are typically required. The laser source for frequency quadrupling should be simple and compact, needs to operate in a fundamental spatial mode to allow efficient nonlinear conversion and should have high efficiency to minimize heat generation and extend battery-powered operational time. Its emission linewidth should be smaller than the phase-matching acceptance bandwidth of nonlinear crystals to be used in frequency quadrupling. In addition, narrow FHG linewidth is required to maximize the spectral resolution of the UV Raman detection system. In this paper we describe, what is to our knowledge, the first compact, high power, efficient deep UV source that meets power, center wavelength and linewidth requirements of portable Raman spectroscopy systems for stand-off detection of explosive materials.
Although multi-Watt, deep-UV powers have been demonstrated with several approaches [5, 6, 8, 10, 13–15], these were too complex and bulky to be usable in a portable Raman detection system. Frequency quadrupling of Nd:YAG microchip laser output [4, 7] generated up to 66 mW at 266 nm , but this wavelength is not ideal due to competing fluorescence that starts at wavelengths longer than ∼270 nm in many materials [1, 2, 17]. To meet all of the requirements for a portable Raman system UV source, we implemented a passively Q-switched Yb:YAG laser [18–23] emitting at 1030 nm. Compared with the 266 nm emission, the 257 nm fourth harmonic of Yb:YAG significantly expands the fluorescence-free detection window for Raman Stokes-shifted lines from 500 cm−1 to ∼2000 cm−1, allowing fluorescence-free detection of Raman lines generated by almost all of the explosive and precursor materials of interest . There are also applications for direct 1030 nm emission from a compact Q-switched Yb:YAG lasers, such as laser detection and ranging (LADAR) and laser range finding (LRF) systems, which often require peak pulse powers in the hundreds of kilowatts range and pulse energies of 0.1–1 mJ. Although such pulses can be generated with laser-diode-seeded fiber master oscillator power amplifier (MOPA) systems using large-core fibers or rod-like photonics-crystal fibers [12–14], these require multiple amplification stages with a large number of components, and are too complex and bulky for use in compact systems.
2. Passively Q-switched Yb:YAG laser
Figure 1 shows the optical arrangement of the passively Q-switched, end-pumped Yb:YAG laser. The physical length of the laser cavity was 15 mm, and the 2.5 mm long, 1×4 mm cross-section Yb:YAG (10 at%) crystal was mounted in an air-cooled copper heat-sink using indium foil. The flat-polished pump face of the crystal was coated for high reflectivity (R>99%) at 1030 nm and high transmission (T=98%) at 940 nm, while the opposite side was anti-reflection (AR) coated for 1030 nm. The fiber-pigtailed (105 μm, 0.15 NA) pump laser generated up to 13 W at 940 nm and was mounted on a temperature-controlled heatsink. We measured a maximum Yb:YAG absorption of 86%, at the optimal pump diode wavelength, and under low pump power conditions (to eliminate absorption bleaching effects). A compact lens pair imaged the output face of the fiber pigtail into the Yb:YAG crystal with a magnification of 1.75, resulting in a minimum pump spot size of 184 μm in the crystal. The longitudinal location of the pump-fiber image inside the crystal was adjusted experimentally to maximize laser output power. In order to achieve narrow laser emission linewidth, a volume Bragg grating (VBG) with 1030 nm center wavelength, 0.12 nm full-width half-maximum (FWHM) bandwidth and R=37% reflectivity was used as the laser output coupler [18, 24, 25]. Linear output polarization was achieved by an air-gap YAG Brewster polarizer, and for Q-switched operation, a Cr4+:YAG saturable absorber with T0=89% non-saturated transmission was placed between the polarizer and the VBG. All intra-cavity surfaces, except the Brewster faces, were AR coated for 1030 nm. The VBG output coupler reflectivity and Cr4+:YAG transmission values were chosen to achieve Q-switched pulse powers adequate for efficient FHG, yet low enough to keep intra-cavity fluence below coating damage levels. The high T0 value also results in a low unsaturated transmission loss and minimizes laser threshold, both required to maintain high laser optical efficiency.
Figure 2 shows laser average output power, for Q-switched and CW (Cr4+:YAG removed) operation, as a function of pump power incident on the Yb:YAG crystal. The Yb:YAG heat-sink temperature was 22°C, and the pump diode temperature was adjusted to maintain a constant pump wavelength for all laser diode currents. Optical-to-optical slope and maximum conversion efficiencies were 44%, and 30%, respectively, for both Q-switched and CW operation. As expected, the high transmission saturable absorber caused only a small increase in the laser threshold and a small decrease in the overall laser efficiency.
As shown in Fig. 3, the Q-switched laser pulse repetition frequency (PRF) varied linearly with pump power, reaching a maximum of 14.5 kHz. The Q-switched pulse energy, calculated from measured average powers and PRFs, was approximately 250 μJ under all pump powers in Fig. 3. Using a measured pulse FWHM duration of 2.5 ns, and assuming a near-Gaussian pulse shape, we calculate a peak pulse power of approximately 100 kW. Significantly higher peak powers of >300 kW were achieved with a T0=80–85% Cr4+:YAG Q-switch in a similar laser configuration, but without a polarizer and with a conventional output coupler (R=30%) instead of a VBG. When a VBG output coupler was used, however, such high peak powers were found to cause damage to some of the intra-cavity AR coated surfaces.
The Q-switched laser emission spectrum (Fig. 4), measured under maximum power conditions, exhibited a linewidth of 0.1 nm (spectrometer resolution was 0.01nm). For comparison, Fig. 4 also shows the 0.5 nm wide emission spectrum of the same laser but with the VBG output coupler removed and replaced with conventional (flat, R=30%) output coupler. The periodic structure of the 0.5 nm spectrum is attributed to residual Fabry-Perot resonances formed by the parallel faces of the Yb:YAG crystal. While the 0.1 nm linewidth of the VBG laser was below the 0.14 nm/cm acceptance bandwidth of the BBO crystals, the non-VBG laser spectrum significantly exceeded this value.
The laser mode intensity distribution, measured by a knife-edge scan of a magnified image of the laser mode within the Yb:YAG, was found to have a near-ideal Gaussian intensity profile with a 1/e2 diameter of 160 μm. A knife-edge scan of the laser far-field distribution yielded a 1/e2 full-angle beam divergence of 10.5 mRad. The resulting beam diameter-divergence product of 1.7 mm-mRad is 1.3 times the value calculated for an ideal Gaussian beam.
3. Fourth harmonic generation
Second harmonic generation (SHG) of Yb:YAG emission was carried out in a 10 mm long lithium triborate (LBO) crystal, 515 nm/1030 nm AR coated, and aligned for critical phase matching (Type-I (oo→e), XY cut, θ=90°, ϕ=13.6°, deff =0.83pm/V). A 38 mm focal length lens imaged and de-magnified the 1030 nm laser near-field output into the LBO crystal. With the lens and crystal position adjusted for maximum SHG conversion efficiency, the 1030 nm focused spot incident in LBO (with LBO angle detuned to eliminate second harmonic generation) was measured using a knife scan method. It was found to have a near-ideal Gaussian intensity profile with a 1/e2 diameter of 61 μm. The 515 nm SHG near-field intensity distribution exiting the LBO, measured with a knife-edge scan and direct imaging, was found to be 47 μm and 95 μm wide (1/e2) along y and x axis, respectively. The larger horizontal width can be attributed to the 8 mRad  LBO spatial walk-off angle, which stretches the SH emission along the horizontal axis. The far-field distribution of the 515 nm beam exhibited a vertical-to-horizontal divergence aspect ratio of 2:1, matching the horizontal-to-vertical aspect ratio of the 515 nm spot exiting LBO.
To generate fourth harmonic of the fundamental emission, we used a 7 mm long beta-barium borate (BBO) crystal (Type-I (oo→e), θ=50.0°, deff =1.7pm/V, 257.5nm/515nm AR). To minimize temperature gradients , the crystal was mounted in a copper block using indium foil. An AR-coated, 25.4mm focal length lens generated a magnified image of the 515 nm LBO output in the BBO crystal. A magnification factor of 2X was found to yield highest 515 nm-to-257 nm conversion efficiency. The intensity distribution of 515 nm light entering BBO, shown in Fig. 5, measured 95 μm vertically and 190 μm (1/e2 points) horizontally. The effects of the spatial walk-off in LBO can be clearly seen in the asymmetric horizontal intensity profile that stretches the right side of the distribution.
To characterize the 257 nm BBO emission, we measured its near-field and far-field intensity distributions. The near-field was observed by generating a magnified image of the output face of BBO and recording it with a Silicon camera, with the results shown in Fig. 6. Vertical and horizontal 1/e2 widths of the near-field distribution were measured to be approximately 700 μm and 130 μm, respectively. The large elongation of the output spot along the vertical axis is a consequence of 85 mRad spatial walk-off angle in BBO . For a 7 mm long crystal, the UV light generated at the input end of BBO is predicted to shift vertically by 600 μm as it propagates to the output end of the crystal, in reasonable agreement with our measurement. The top of the intensity distribution of Fig. 6 corresponds to UV emission generated near the input end of BBO; it undergoes largest vertical displacement and has highest intensity since the 515 nm pump beam is at maximum power when it first enters the crystal. A monotonic decrease in the UV intensity toward the bottom of the trace is due to pump depletion along the propagation direction, caused by its conversion to 257 nm. A slight bump at the tail end of the distribution of Fig. 6 is attributed to residual leakage of the 515 nm light through the dichroic filters and high camera sensitivity at this wavelength.
To measure the far-field angular distribution of the 257 nm emission, we placed a 1m focal length lens in the beam, and recorded the focal plane intensity distribution, shown in Fig. 7. Far-field divergence angles were 0.8 mRad and 2.5 mRad (between 1/e2 intensity points) in y and x directions, respectively. For purposes of comparison, the divergence angles for an ideal Gaussian beam with y and x-axis 1/e2 diameters of 700 μm and 130 μm, are 0.5 mRad and 2.5 mRad.
Figure 8 shows the 1030 nm fundamental power incident on the LBO, the 515 nm SH power generated by LBO, and 257 nm FH power generated by BBO, plotted as a function of laser diode pump power incident on the Yb:YAG. As expected from the nearly-constant Q-switched peak power for all laser powers, both 515 nm and 257 nm powers grow linearly with the fundamental power, reaching a maximum of 2.5 W and 1.1 W respectively. Frequency conversion efficiencies, shown in Fig. 9, were 70% for the 1030-to-515 nm conversion, 45% for the 515-to-257 nm conversion, and 31% for an overall 1030-to-257 nm conversion, and were near-constant for most of the pump power range. These conversion efficiencies are similar to the highest reported for UV FHG generation [6–8, 12–15]. With a diode electrical power efficiency of 50%, the overall UV source electrical efficiency is calculated to be approximately 5%, which to our knowledge is the highest reported.
Temporal pulse shapes for the 1030 nm, 515 nm and 257 nm, measured using a 6 GHz bandwidth detector, are shown in Fig. 10 for maximum pump power condition. The 1030 nm pulse FWHM varied from an average of 2.4 ns at lowest pump power to 2.5 ns at the highest pump power; the FWHM of the SHG and FHG pulses was ∼2.1 ns. The relatively small reduction in the pulse-width for the harmonics relative to the fundamental is attributed to the high conversion efficiencies, causing clamping of the peak of the pulse. The 1030 nm pulse train, also shown in Figure 10, was recorded at maximum pump power, and exhibits pulse-to-pulse energy variation of approximately 5%.
We demonstrated a high power, narrow-band, compact deep UV source, generating 1.1 W average power at 257 nm. A 31% overall infrared-to-UV FHG harmonic conversion efficiency was achieved using LBO and BBO crystals, resulting in a 5% electrical efficiency for the UV source. Narrow-band 0.1 nm emission linewidth at 1030 nm and 0.025 nm at 257 nm, required for high resolution Raman spectroscopy, was realized through the use of a VBG laser output coupler in the passively Q-switched Yb:YAG laser. The deep-UV source described is well suited for portable Raman detection systems, as well as other applications, such as material processing, that can benefit from its relatively simple and compact configuration.
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